I don't think there are any rules for nick names. You just use any nick name you want, even if it has never been done before.
Of course, people might not get that the formal name for
Manny is
Amanda, but I don't think that matters.
"But it’s all right now.
I learned my lesson well.
You see you can’t please everyone
So you got to please yourself."
Rick Nelson,
Garden Party
"It does not become me to make myself smaller than I am." (
Edith Södergran 1891-1923)